This invention relates to metal detectors, and specifically to receiver coils used in pulse-induction-type metal detectors.
In such detectors, a primary magnetic flux pulse is directed into the medium to be searched, and a secondary flux is generated by eddy-currents in the target. The voltage induced by the secondary flux is sensed by a receiver coil.
The primary flux also induces a voltage in the receiver coil, and the target voltage cannot be effectively detected until the effects of the primary flux have substantially subsided.
The energy absorbed by the receiver coil is not dissipated immediately after the cessation of the coil pulse. The inductance and self-capacitance of the receiver coil constitute a resonant circuit that oscillates until the absorbed energy is dissipated. Usually a damping circuit is placed across the terminals of the coil to minimize the oscillations.
Even when a damping circuit is used, a minimum delay must be inserted between the end of the coil pulse and the beginning of the signal gating pulse, to avoid admixture of target and artifact signals.
The length of the delay determines the shortest time constant that a target signal can have to be detectable by the coil system. The target signal decays exponentially with time, and after a time interval that corresponds to about four time constants, it has been reduced to substantially zero.
This circumstance is of particular significance in metal detectors intended to detect high-resistivity metals. Tramp metal on conveyor belts frequently includes objects made of high-resistivity alloys. Gold veins and nuggets also represent targets with short time constants, owing to the relatively high resistivity of gold and the irregular eddy-current paths in such targets.
To mitigate the above difficulties, coil configurations that minimize the amount of intercepted energy have been devised. The “induction balance” type coil configuration is an example of such an arrangement. This amounts to a physical alignment of the coils in which the receiver coil intercepts essentially equal amounts of flux of opposing polarities from the transmitter coil.
The same effect can be obtained by adding a compensating coil with a tighter coupling to the receiver coil than the transmitter coil.
Penland, in U.S. Pat. No. 3,471,773 shows a coil configuration exemplifying the induction balance principle.
Thompson, in U.S. Pat. No. 4,255,711 shows the use of the compensating coil. Both of the above methods suffer from the drawback that the sensitivity of the detector is decreased by their use.
Furthermore, the balanced condition of the coil system is disturbed by the presence of magnetic minerals in the searched medium.
Parks, in U.S. Pat. No. 4,866,424 recognizes the problem created by energy absorption by the receiver coil, but does not solve the problem. Commercialized units of the detector described in the patent are being used at airports, and field tests have shown that knives made from titanal with 6-inch blades can be passed through the coil system without causing an alarm, even when the detector is used at its highest sensitivity